Image display medium, process for forming image, and multicolor image-forming apparatus

ABSTRACT

An image display medium that includes a photoconductive layer containing a photochromic compound and an electron accepting compound; and a substrate, in which the photochromic compound contains a fulgide compound, and the electron accepting compound contains a compound selected at least from: a) a phosphonic acid compound having analiphatic group containing 12 or more carbon atoms; b) an aliphatic carboxylic acid compound having an aliphatic group containing 12 or more carbon atoms; and c) a phenolic compound having an aliphatic group containing 12 or more carbon atoms, a process for forming an image using the medium, and a multicolor image-forming apparatus using the medium and suitable for the process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image display medium, a process forforming images, and a multicolor image-forming apparatus. Morespecifically, it relates to an image display medium, a process forforming images, and a multicolor image-forming apparatus that canrepeatedly form color images upon light irradiation.

2. Description of the Related Art

Erasable or rewritable image display media using photochromic compoundsthat can reversibly change their colors upon irradiation with light havebeen proposed. However, practical processes and apparatus that canrepeatedly rewrite full-color images have not yet been proposed.

For example, Japanese Patent Application Laid-Open (JP-A) No. 05-271649discloses, as a process for forming multicolor images using aphotochromic compound, a process of applying ultraviolet radiation withdistinct corresponding wavelengths to a mixture of three diarylethenes,where the diarylethenes include one turning golden yellow uponirradiation with ultraviolet radiation wavelength of 254 nm, one turningred upon irradiation with ultraviolet radiation wavelength of 313 nm,and one turning bluish purple (violet) upon irradiation with ultravioletradiation wavelength of 365 nm.

To form full-color images, three or more photochromic compoundsdeveloping three primary colors (blue, green, and red, or yellow,magenta, and cyan) must be controlled by light to develop and/or reducetheir colors. The aforementioned process requires that wavelengths ofthe ultraviolet radiation determine whether or not each of the materialcompounds develops its color. In other words, the process requires threeor more photochromic compounds showing absorption in ultravioletwavelengths, where each of the wavelengths does not overlap one another,and that these photochromic compounds show the three primary colors uponcolor development. However, no compounds comprising such compounds hasbeen found in reality. To use in practice, materials must havesatisfactory repetition durability, heat and moisture resistance inaddition to color developing properties. Such materials satisfying allthe requirements are very difficult to develop.

Japanese Patent Application Laid-Open UP-A) No. 07-199401 proposes aprocess for forming color images. In this process, ultraviolet radiationof 366 nm is applied to three photochromic fulgide compounds turningyellow, magenta, and cyan upon color development, respectively, tothereby make all the fulgide compounds to develop their colors. Whitelight is then applied to the fulgide compounds through a color positivefilm to selectively reduce the colors of individual photochromic fulgidecompounds according to necessity to thereby form color images. Thisprocess only requires one ultraviolet light source, but requires apreparation of a color positive film of the target image upon each useand is not practical for use in color image output in office systems.

These and other proposals for rewritable image media and processes usingphotochromic compounds are not satisfactorily practical in the formationof color images.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an imagedisplay medium rewritable upon irradiation with light and a process forforming images using the same, which can shorten a time period for imageformation and can improve persistence of colors of formed images.Another object of the present invention is to provide an image displaymedium and a process for forming images which can give high-definitionand high-quality color images.

A further object of the present invention is to provide high-definitionand high-quality multicolor image-forming apparatus, which is capable ofcontrolling decolorization sensitivity of an image display mediumrewritable upon irradiation with light and is capable of shortening thetime period for image formation and of improving persistence of coloringstability of formed images. More specifically, the further object of thepresent invention is to provide a multicolor image-forming apparatus,which can switch the magnitude of the decolorization sensitivity in adecolorizing process in the formation of images.

Specifically, the present invention provides, in a first aspect, animage display medium which comprises a photoconductive layer containinga photochromic compound and an electron accepting compound, and asubstrate, in which the photochromic compound contains a fulgidecompound, and the electron accepting compound contains a compoundselected at least from a) a phosphoric acid compound having an aliphaticgroup containing 12 or more carbon atoms, b) an aliphatic carboxylicacid compound having an aliphatic group containing 12 or more carbonatoms, and c) a phenol compound having an aliphatic group containing 12or more carbon atoms.

By including the fulgide compound as the photochromic compound and thespecific electron accepting compound in the photoconductive layer, thepresent invention can provide a rewritable image display medium that canreversibly change the decolorization sensitivity, can shorten the imageforming time period and can improve persistence of coloring stability offormed images.

According to a second aspect of the present invention, theelectron-accepting compound may be a phosphoric acid compoundrepresented by the following Formula (I):

where R¹ is an aliphatic, group containing 12 or more carbon atoms.

According to a third aspect of the present invention, the electronaccepting compound may be an alpha-hydroxyalphaticcarboxylic acidcompound represented by the following Formula (II):

where R² is an aliphatic group containing 12 or more carbon atoms.

According to a fourth aspect of the present invention, the electronaccepting compound may be an aliphatic carboxylic acid compoundcontaining 12 or more carbon atoms, and at least one of the carbon atomsis bonded to a halogen element at one of an alpha-position and abeta-position of the aliphatic carboxylic acid compound.

According to a fifth aspect of the present invention, the electronaccepting compound may be an aliphatic carboxylic acid compoundcontaining 12 or more carbon atoms, and the aliphatic carboxylic acidcompound has at least one oxo group at any one of an alpha-position, abeta-position, and a gamma-position thereof.

According to a sixth aspect of the present invention, the electronaccepting compound may be an aliphatic carboxylic acid compoundrepresented by the following Formula (III):

where R³ is an aliphatic group containing 12 or more carbon atoms; “X”is one of an oxygen atom and a sulfur atom; and “n” is 1 when “X” is anoxygen atom, and “n” is one of 1 and 2 when “X” is a sulfur atom.

According to a seventh aspect of the present invention, the electronaccepting compound may be an aliphatic carboxylic acid compoundrepresented by the following Formula (IV):

where R⁴, R⁵, and R⁶ are identical or different, and each express one ofa hydrogen atom and an aliphatic group, in which at least one of R⁴, R⁵,and R⁶ is an aliphatic group containing 12 or more carbon atoms.

According to an eighth aspect of the present invention, the electronaccepting compound may be an aliphatic carboxylic acid compoundrepresented by the following Formula (V):

where R⁷ and R⁸ are identical or different, and each express one of ahydrogen atom and an aliphatic group, in which at least one of R⁷ and R⁸is an aliphatic group containing 12 or more carbon atoms.

According to a ninth aspect of the present invention, the electronaccepting compound may be an aliphatic carboxylic acid compoundrepresented by the following Formula (VI):

where R⁹ is an aliphatic group containing 12 or more carbon atoms; “n”is 0 or 1; and “m” is an integer from 1 to 3, in which “m” is 2 or 3when “n” is 0, and “m” is 1 or 2 when “n” is 1.

According to a tenth aspect of the present invention, the electronaccepting compound may be a phenolic compound represented by thefollowing Formula (VII):

where R¹⁰ is an aliphatic group containing 12 or more carbon atoms; “Y”is one of S, O, CONH, and COO; and “n” is an integer from 1 to 3.

According to an eleventh aspect of the present invention, thephotochromic compound may contain two or more fulgide compounds havingdifferent maximum absorption wavelengths, when the photochromic compoundis colored.

According to a twelfth aspect of the present invention, two or more ofthe fulgide compounds may contain a fulgide compound (A) having amaximum absorption wavelength of 400 nm or more and less than 500 nm,when the photochromic compound is colored; a fulgide compound (B) havinga maximum absorption wavelength of 500 nm or more and less than 600 nm,when the photochromic compound is colored; and a fulgide compound (C)having a maximum absorption wavelength of 600 nm or more and less than700 nm, when the photochromic compound is colored.

According to a thirteenth aspect of the present invention, thephotoconductive layer may have a plurality of layers, and each of thelayers may have fulgide compounds having different maximum absorptionwavelengths, when the photochromic compound is colored.

According to a fourteenth aspect of the present invention, the fulgidecompounds having different maximum absorption wavelengths when thephotochromic compound is colored may contain a fulgide compound (A)having a maximum absorption wavelength of 400 nm or more and less than500 nm when the photochromic compound is colored; a fulgide compound (B)having a maximum absorption wavelength of 500 nm or more and less than600 nm when the photochromic compound is colored; and a fulgide compound(C) having a maximum absorption wavelength of 600 nm or more and lessthan 700 nm when the photochromic compound is colored.

According to a fifteenth aspect of the present invention, thephotoconductive layer may further have an intermediate layer between aplurality of the layers.

According to a sixteenth aspect of the present invention, the imagedisplay medium may further comprise a protecting layer on a surface ofthe photoconductive layer.

The present invention provide, in a seventeenth aspect, a process forforming an image which comprises the steps of; irradiating ultravioletradiation to an image display medium so as to color all types ofphotochromic compounds contained in a photoconductive layer; heating theimage display medium to a melting point of an electron acceptingcompound or higher; irradiating visible radiation to a desired portionof the image display medium at a wavelength corresponding to the maximumabsorption wavelength of each of the photochromic compounds, so as toselectively decolorize the photochromic compounds; and heating the imagedisplay medium to the melting point of the electron accepting compoundor lower, in which the image display medium comprises a photoconductivelayer containing a photochromic compound and an electron acceptingcompound; and a substrate, where the photochromic compound contains afulgide compound, and the electron accepting compound contains acompound selected at least from a) a phosphonic acid compound having analiphatic group containing 12 or more carbon atoms; b) an aliphaticcarboxylic acid compound having an aliphatic group containing 12 or morecarbon atoms; and c) a phenolic compound having an aliphatic groupcontaining 12 or more carbon atoms.

By using the image display medium containing the photochromic fulgidecompound and the specific electron accepting compound in thephotoconductive layer and by including heating steps for reversiblychanging the decolorization sensitivity, the present invention canprovide a process for forming rewritable multicolor images upon lightirradiation, in which the process enables shorter time period forforming an image and also enables improving persistence of developedcolors of formed images.

According to an eighteenth aspect of the present invention, the imagedisplay medium is heated at a melting point of the fulgide compound orhigher, in the step of heating the image display medium to a meltingpoint of an electron accepting compound or higher.

According to a nineteenth aspect of the present invention, the processmay further comprise a step of rapidly cooling the image display medium,after the step of heating the image display medium to a melting point ofan electron accepting compound or higher.

The present invention provides, in a twentieth aspect, a multicolorimage-forming apparatus which comprise an ultraviolet radiationirradiator configured to irradiate ultraviolet radiation to the imagedisplay medium of the present invention so as to color all types ofphotochromic compounds in a photoconductive layer; a first heaterconfigured to temporarily heat the image display medium; a visibleradiation irradiator configured to irradiate visible radiation to adesired portion of the image display medium, at a wavelengthcorresponding to the maximum absorption wavelength of the photochromiccompounds in a state of colored, so as to selectively decolorize thephotochromic compounds; and a second heater configured to temporarilyheat the image display medium after irradiating the visible radiation,in which the image display medium repeatedly forms a multicolor image.

The apparatus can control the decolorization sensitivity, can shortenthe image forming time period and can ensure satisfactory persistence ofdeveloped colors of formed images.

According to a twenty first aspect of the present invention, themulticolor image-forming apparatus may further comprise a transferconfigured to transfer the image display medium, in which the imagedisplay medium relatively moves towards the ultraviolet radiationirradiator, the first heater, the visible radiation irradiator, and thesecond heater, in this order.

By these configurations, the multicolor image-forming apparatus canautomatically move the image display medium inside the apparatus whichincludes an ultraviolet radiation irradiator, the first heater, thevisible radiation irradiator, and the second heater. The multicolorimage-forming apparatus can also control or change the position of theimage display medium as required at each of the heaters and theirradiators, and thereby has much improved operability.

According to a twenty second aspect of the present invention, themulticolor image-forming apparatus may further comprise an inlet and anoutlet, in which the image display medium is inserted from the inlet,the image display medium is automatically transferred inside themulticolor image-forming apparatus, and the image display medium isejected from the outlet.

By these configurations, the multicolor image-forming apparatus enablesimproving the usage, and can have highly improved abilities at each ofthe steps.

The present invention further provides, in a twenty third aspect, amulticolor image forming apparatus which comprises an ultravioletradiation irradiator configured to irradiate ultraviolet radiation tothe image display medium of the present invention so as to color alltypes of photochromic compounds contained in a photoconductive layer; avisible radiation irradiator configured to irradiate visible radiationto a desired portion of the image display medium, at a wavelengthcorresponding to the maximum absorption wavelength of the photochromiccompounds in a state of colored, so as to selectively decolorize thephotochromic compounds, when the photochromic compounds are colored; anda heater configured to temporarily heat the image display medium, wherethe image display medium repeatedly forms a multicolor image, and theheater heats the image medium both before and after irradiating visibleradiation, after irradiating ultraviolet radiation.

According to a twenty fourth aspect of the present invention, themulticolor image-forming apparatus may further comprise a transferconfigured to transfer the image display medium, where the image displaymedium relatively moves towards the ultraviolet radiation irradiator,the heater, and the visible radiation irradiator, in this order, and,the image display medium moves back to the heater.

According to a twenty fifth aspect of the present invention, themulticolor image-forming apparatus may further comprise an inlet/outletport which works as an inlet to insert an image display medium and as anoutlet to eject the image display medium, in which the image displaymedium is automatically transferred from the inlet/outlet to inside themulticolor image-forming apparatus, and the image display medium isejected from the inlet/outlet.

According to a twenty sixth aspect of the present invention, themulticolor image-forming apparatus may further comprise a visibleradiation irradiator configured to irradiate white light to the imagedisplay medium so as to decolorize an entire portion of the imagedisplay medium.

The twenty sixth aspect provides a multicolor image-forming apparatuswhich can entirely decolorize the image display medium in a short periodof time, when requested.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views showing an example of forming animage by light irradiation;

FIG. 2 is a graph schematically showing an example of absorption offulgide compounds when the fulgide compounds are colored in the visiblerange of wavelengths;

FIG. 3 is a schematic view showing an example of a multicolorimage-forming apparatus of the present invention;

FIG. 4 is a schematic view showing an example of a multicolorimage-forming apparatus of the present invention, which has aninlet/outlet port;

FIG. 5 is a schematic view showing an example of a multicolorimage-forming apparatus of the present invention, which has a whiteradiation irradiator;

FIG. 6 is a schematic view showing an example of a multicolorimage-forming apparatus of the present invention, which has aninlet/outlet port and a white radiation irradiator;

FIG. 7 is a graph showing an example of changes in reflection with time,in which visible radiation is irradiated;

FIG. 8 is a graph showing an example of changes in reflection ratio at abottom wavelength;

FIG. 9 is a graph showing an example of light-emitting properties atwavelengths of a visible radiation irradiator used in the examples; and

FIG. 10 is a schematic view showing an example of the visible radiationirradiator used in the examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Image Display Media and Processes for Forming Images)

The image display medium of the present invention is an image displaymedium comprising a photoconductive layer containing at least onephotochromic compound and at least one electron accepting compounddisposed on a substrate, in which the photochromic compound is at leastone fulgide compound, and the electron accepting compound is at leastone selected from phosphonic acid compounds, aliphatic carboxylic acidcompounds, and phenolic compounds each having an aliphatic groupcontaining 12 or more carbon atoms.

The fulgide compounds used in the present invention include fulgidecompounds represented by following Formula (VIII) and Formula (IX):

wherein R¹¹, R¹², R¹³, and R¹⁴ are identical or different and are eachone of a hydrogen atom, alkyl groups, alkoxy groups, aromatic rings, andheteroaromatic rings, where at least one of R¹¹, R¹², R¹³, and R¹⁴structurally includes an aromatic ring or a heteroaromatic ring, fulgidecompounds represented by following Formula (IX):

wherein R¹⁵, R¹⁶, R¹⁷, R¹⁸, and R¹⁹ are identical or different and areeach one of a hydrogen atom, alkyl groups, alkoxy groups, aromaticrings, and heteroaromatic rings, where at least one of R¹⁵, R¹⁶, R¹⁷,R¹⁸, and R¹⁹ structurally includes an aromatic ring or a heteroaromaticring, and other compounds that structurally include these compounds andshow photochromism.

The decolorization sensitivity of fulgide compounds will be describedbelow.

The decolorization sensitivity directly depends on quantum efficiency(φCE) in a decolorizing reaction of a photochromic compound, and controlof the decolorization sensitivity of a compound substantially meanscontrol of φCE of the compound. Hereinafter, changes in decolorizationsensitivity mean changes in φCE of the compound in question.

The decolorization sensitivity of such a fulgide compound represented byFormula (VIII) or (IX) generally significantly depends on electronicproperties (electron donating property or electron accepting property)due to the chemical structure of an aromatic ring moiety of thecompound. For example, a compound having an aromatic ring with highelectron donating property has a low decolorization sensitivity, and onehaving an aromatic ring with low electron donating property has a highdecolorization sensitivity.

The decolorization sensitivity of a compound can also vary depending onelectronic properties of a medium surrounding the compound. In otherwords, the electronic properties of the aromatic ring moiety of thefulgide compound apparently vary by the interaction between the compoundand the medium. When the interaction between an electron acceptingcompound in a medium and the fulgide compound is large, the electrondonating property of the aromatic ring moiety decreases, to therebyincrease the decolorization sensitivity. In contrast, when theinteraction is small, the electron donating property of the aromaticring moiety increase to thereby decrease the decolorization sensitivity.Accordingly, the decolorization sensitivity of a fulgide compound can becontrolled by controlling the interaction between the fulgide compoundand an electron accepting compound used in combination.

Such electron accepting compounds for use in the present invention arecompounds that can change the electronic properties of an aromatic ringmoiety of a fulgide compound and are selected from the group consistingof phosphonic acid compounds, carboxylic acid compounds, and phenoliccompounds. These compounds should each have an aliphatic chain structure(group) containing 12 or more carbon atoms to control intermolecularcohesion of the electron accepting compound. Such aliphatic groupsinclude straight- or branched-chain alkyl groups and alkenyl groups,each of which may have at least one substituent such as halogen atoms,alkoxy groups, ester groups, or the like.

Examples of the electron accepting compounds are as follows.

The phosphonic acid compounds represented by Formula (I) include, butare not limited to, dodecylphosphonic acid, tetradecylphosphonic acid,hexadecylphosphonic acid, octadecylphosphonic acid, eicosylphosphonicacid, docosylphosphonic acid, tetracosylphosphonic acid,hexacosylphosphonic acid, octacosylphosphonic acid, and the like.

The aliphatic alpha-hydroxycarboxylic acid compounds represented byFormula (II) include, but are not limited to, alpha-hydroxydodecanoicacid, alpha-hydroxytetradecanoic acid, alpha-hydroxyhexadecanoic acid,alpha-hydroxyoctadecanoic acid, alpha-hydroxypentadecanoic acid,alpha-hydroxyeicosanic acid, alpha-hydroxydocosanoic acid,alpha-hydroxytetracosanoic acid, alpha-hydroxyhexacosanoic acid,alpha-hydroxyoctacosanoic acid, and the like.

The aliphatic carboxylic acid compounds each containing 12 or morecarbon atoms and having a halogen element bonded to at least one ofcarbon atoms at the alpha-position and the beta-position include, butare not limited to, 2-bromohexadecanoic acid, 2-bromoheptadecanoic acid,2-bromooctadecanoic acid, 2-bromoeicosanic acid, 2-bromodocosanoic acid,2-bromotetracosanoic acid, 3-bromooctadecanoic acid, 3-bromoeicosanicacid, 2,3-dibromooctadecanoic acid, 2-fluorododecanoic acid,2-fluorotetradecanoic acid, 2-fluorohexadecanoic acid,2-fluorooctadecanoic acid, 2-fluoroeicosanic acid, 2-fluorodocosanoicacid, 2-iodohexadecanoic acid, 2-iodooctadecanoic acid,3-iodohexadecanoic acid, 3-iodooctadecanoic acid, perfluorooctadecanoicacid, and the like.

The aliphatic carboxylic acid compounds each containing 12 or morecarbon atoms and having at least one oxo group bonded to at least onecarbon atom at the alpha-position, the beta-position, or thegamma-position include, but are not limited to, 2-oxododecanoic acid,2-oxotetradecanoic acid, 2-oxohexadecanoic acid, 2-oxooctadecanoic acid,2-oxoeicosanic acid, 2-oxotetracosanoic acid, 3-oxododecanoic acid,3-oxotetradecanoic acid, 3-oxohexadecanoic acid, 3-oxooctadecanoic acid,3-oxoeicosanic acid, 3-oxotetracosanoic acid, 4-oxohexadecanoic acid,4-oxooctadecanoic acid, 4-oxodocosanoic acid, and the like.

The aliphatic carboxylic acid compounds represented by Formula (III)include, but are not limited to, 2-(dodecyloxy)succinic acid,2-(tetradecyloxy)succinic acid, 2-(hexadecyloxy)succinic acid,2-(octadecyloxy)succinic acid, 2-(eicosyloxy)succinic acid,2-(docosyloxy)succinic acid, 2-(tetracosyloxy)succinic acid,2-(dodecylthio)succinic acid, 2-(tetradecylthio)succinic acid,2-(hexadecylthio)succinic acid, 2-(octadecylthio)succinic acid,2-(eicosylthio)succinic acid, 2-(docosylthio)succinic acid,2-(tetracosylthio)succinic acid, 2-(dodecyldithio)succinic acid,2-(tetradecyldithio)succinic acid, 2-(hexadecyldithio)succinic acid,2-(octadecyldithio)succinic acid, 2-(eicosyldithio)succinic acid,2-(docosyldithio)succinic acid, 2-(tetracosyldithio)succinic acid, andthe like.

The aliphatic carboxylic acid compounds represented by Formula (IV)include, but are not limited to, dodecylsuccinic acid, tridecylsuccinicacid, tetradecylsuccinic acid, pentadecylsuccinic acid,octadecylsuccinic acid, eicosylsuccinic acid, docosylsuccinic acid,2,3-dihexadecylsuccinic acid, 2,3-dioctadecylsuccinic acid,2-methyl-3-dodecylsuccinic acid, 2-methyl-3-tetradecylsuccinic acid,2-methyl-3-hexadecylsuccinic acid, 2-ethyl-3-dodecylsuccinic acid,2-propyl-3-dodecylsuccinic acid, 2-octyl-3-hexadecylsuccinic acid,2-tetradecyl-3-octadecylsuccinic acid, and the like.

The aliphatic carboxylic acid compounds represented by Formula (V)include, but are not limited to, dodecylmalonic acid, tetradecylmalonicacid, hexadecylmalonic acid, octadecylmalonic acid, eicosylmalonic acid,docosylmalonic acid, tetracosylmalonic acid, didodecylmalonic acid,ditetradecylmalonic acid, dihexadecylmalonic acid, dioctadecylmalonicacid, dieicosylmalonic acid, didocosylmalonic acid,methyloctadecylmalonic acid, methyleicosylmalonic acid,methyldocosylmalonic acid, methyltetracosylmalonic acid,ethyloctadecylmalonic acid, ethyleicosylmalonic acid,ethyldocosylmalonic acid, ethyltetracosylmalonic acid, and the like.

The aliphatic carboxylic acid compounds represented by Formula (VI)include, but are not limited to, 2-dodecylglutaric acid,2-hexadecylglutaric acid, 2-octadecylglutaric acid, 2-eicosylglutaricacid, 2-docosylglutaric acid, 2-dodecyladipic acid, 2-pentadecyladipicacid, 2-octadecyladipic acid, 2-eicosyladipic acid, 2-docosyladipicacid, and the like.

The phenolic compounds represented by Formula (VII) include, but are notlimited to, p-(dodecylthio)phenol, p-(tetradecylthio)phenol,p-(hexadecylthio)phenol, p-(octadecylthio)phenol, p-(eicosylthio)phenol,p-(docosylthio)phenol, p-(tetracosylthio)phenol, p-(dodecyloxy)phenol,p-(tetradecyloxy)phenol, p-(hexadecyloxy)phenol, p-(octadecyloxy)phenol,p-(eicosyloxy)phenol, p-(docosyloxy)phenol, p-(tetracosyloxy)phenol,p-dodecylcarbamoylphenol, p-tetradecylcarbamoylphenol,p-hexadecylcarbamoylphenol, p-octadecylcarbamoylphenol,p-eicosylcarbamoylphenol, p-docosylcarbamoylphenol,p-tetracosylcarbamoylphenol, hexadecyl ester gallate, octadecyl estergallate, eicosyl ester gallate, docosyl ester gallate, tetracosyl estergallate, and the like.

The photoconductive layer may further comprise a binder materialaccording to necessity in addition to the fulgide compound and theelectron accepting compound. The binder materials is preferably resinousmaterials that does not adversely affect the photochromic function ofthe fulgide compound, has satisfactory compatibility (miscibility) withthe fulgide compound and the electron accepting compound, can form afilm, and has satisfactory transparency after curing. Examples of thebinder material include polystyrenes, polyesters, polymethylmethacrylate, vinyl chloride-vinylidene chloride copolymer, polyvinylchloride, poly vinylidenechloride, poly vinylacetate, and the like.The materials for the photoconductive layer also include phenoxy resins,aromatic polyesters, phenolic resins, epoxy resins, and the like.

Materials for the substrate include, but are not limited to,poly(ethylene terephthalate), poly(ether sulfone), polycarbonates, andother transparent materials, as well as white or other colored productsof these materials, paper, and other opaque materials.

The proportions of the fulgide compound, the electron acceptingcompound, and the binder material in the photoconductive layer are notspecifically limited and depend on combination of those compounds andthe binder material. The photoconductive layer preferably has 5% to 30%of the fulgide compound, 20% to 80% of the electron accepting compound,and 20% to 50% of the binder material for preferable results.

The photoconductive layer can be formed by coating (applying), vapordeposition and other techniques. Of those, the preferred is coating, asit is simple. The photoconductive layer can be formed, for example, bydissolving the fulgide compound, the electron accepting compound, and,if necessary, the binder material in a solvent, by applying the solutionwith printing, spin coating, or another technique, and drying.

The process for forming images using the image display medium will bedescribed below.

Ultraviolet radiation is applied to the image display medium to therebycause the fulgide compound in the photoconductive layer to color. As alight source for the application of the ultraviolet radiation, asuitable combination of a mercury lamp or a xenon lamp with an opticalfilter can be employed, from which ultraviolet radiation of desiredwavelengths is extracted. Alternatively, light-emitting diodes (LEDs),laser diodes (LDs), and other light-emitting devices each emittingradiation with a specific wavelength can be used.

The image display medium is then temporarily heated to a temperature(hereinafter, may be referred to as “Temperature I”) equal to or higherthan the melting point of the electron accepting compound. By thisprocedure, aliphatic group moieties of the electron accepting compoundaggregate regularly to some extent, and acidic group moieties of theelectron accepting compound closely interact with an aromatic ringmoiety of the fulgide compound and are stabilized (hereinafter referredto as “State A”). Thus, the decolorization sensitivity of the fulgidecompound increases.

In this state, visible radiation is applied to the image display mediumin the decolorizing process. Light sources for the application of thevisible radiation include lamps comprising a white light source and anoptical filter in combination, and LEDs, LDs, and other light-emittingdevices each emitting radiation with a specific wavelength. To irradiatea desired portion alone, for example, the image display medium is movedwith respect to a light source array comprising small light-emittingunits that can be On or Off in light emission and are continuouslyarrayed, and On/Off in light emission of the light-emitting units of thelight source array is respectively controlled.

Thus, by subjecting the image display medium to the decolorizing processwhile the fulgide compound has an increased decolorization sensitivity,the color of the fulgide compound can be reduced with low energy, i.e.,in a short time to thereby form an image.

Next, the image display medium is temporarily heated to a temperature(hereinafter may be referred to as “Temperature II”) equal to or lowerthan the melting point of the electron accepting compound, and theacidic group moieties of the electron accepting compound closelyaggregate with one another with less interaction with the aromatic ringmoieties of the fulgide compound and are stabilized (hereinafterreferred to as “State B”). This procedure may be performed bytemporarily heating the image display medium to a temperature equal toor higher than the melting point of the electron accepting compound andgradually cooling the heated image display medium. Thus, thedecolorization sensitivity of the fulgide compound decreases to avoiddecolorization by, for example, illumination light to thereby improvepersistence of coloring stability of formed images.

When the melting point of the fulgide compound is higher than that ofthe electron accepting compound, Temperature I may be the melting pointof the electron accepting compound or higher, and is preferably equal toor higher than the melting point of the fulgide compound in the step ofheating the image display medium to achieve State A.

After the completion of heating, the image display medium is preferablyrapidly cooled. If it is gradually cooled, State A is more likely tobecome State B. This is because heating temperatures for achieving StateB are lower than those for achieving State A. Accordingly, by heatingthe photoconductive layer in State A to specific temperatures, State Bcan be obtained.

Heating temperatures in the step of heating the image display medium toa melting point of the electron accepting compound or higher (which maybe referred to as a “first heating step,” hereinafter) to achieve StateA, and in the step of heating the image display medium to lower than amelting point of the electron accepting compound (which may be referredto as a “second heating step,” hereinafter) to achieve State B can beappropriately set depending on the types and combination of the fulgidecompound, the electron accepting compound, and, the binder.

Image display media and processes for forming multicolor images will bedescribed.

The image display medium for forming color images comprises aphotoconductive layer disposed on a substrate, which photoconductivelayer comprises two or more fulgide compounds and at least one electronaccepting compound. The two or more fulgide compounds have differentmaximum absorption wavelengths when the fulgide compounds are colored,i.e., have different colors to be seen when the fulgide compounds arecolored. The maximum absorption wavelengths and the types of the fulgidecompounds may be set depending on colors to be displayed and the numberthereof.

FIGS. 1A and 1B are schematic views of image formation upon irradiationwith light, by taking photoconductive layers comprising three fulgidecompounds as an example. With reference to FIG. 1A, threephotoconductive layers 7 a, 7 b, and 7 c comprising different fulgidecompounds are formed on a substrate 8. By applying ultravioletradiation, all the three fulgide compounds contained in thephotoconductive layers of an image display medium as shown in FIG. 1A.FIG. 2 is a graph schematically showing absorption wavelengths of thefulgide compounds contained in the photoconductive layers 7 a, 7 b, and7 c, respectively when the fulgide compounds are colored.

Next, radiation with wavelengths respectively corresponding toabsorption in the visible range of wavelengths of the colored fulgidecompounds (Wavelengths A, B, and C in the vicinity of the maximumabsorption wavelengths shown in FIG. 2) is applied to desired areas. Bythis procedure, corresponding specific fulgide compounds are selectivelydecolorized to yield a desired color image as shown in FIG. 1B.

The two or more fulgide compounds having different maximum absorptionwavelengths when the fulgide compounds are colored preferably comprise afulgide compound (A) having a maximum absorption wavelength of 400 nm ormore and less than 500 nm when the fulgide compounds are colored; afulgide compound (B) having a maximum absorption wavelength of 500 nm ormore and less than 600 nm when the fulgide compounds are colored; and afulgide compound (C) having a maximum absorption wavelength of 600 nm ormore and less than 700 nm when the fulgide compounds are colored. Suchfulgide compounds having these maximum absorption wavelengths showcolors to be seen when the fulgide compounds are colored substantiallycorresponding to yellow, magenta, and cyan and can thereby constitutethe three primary colors.

Examples of the fulgide compound (A) include

-   2-[1-(4-acetyl-2,5-dimethyl-3-furyl)ethylidene]-3-isopropylidenesuccinic    anhydride,-   2-[1-(5-methyl-2-phenyl-4-oxazolyl)ethylidene]-3-isopropylidenesuccinic    anhydride, and the like.

Examples of the fulgide compound (B) include

-   2-[1-(2,5-dimethyl-1-phenylpyrazolyl)ethylidene]-3-isopropylidenesuccinic    anhydride,-   2-[1-(3-methoxy-5-methyl-1-phenyl-4-pyrazolyl)ethylidene]-3-isopropylidenesuccinic    anhydride, and the like.

Examples of the fulgide compound (C) include

-   2-[1-(1,2,5-trimethyl-3-pyrrolyl)ethylidene]-3-isopropylidenesuccinic    anhydride,-   2-[2,6-dimethyl-3,5-bis(p-dimethylaminostyryl)benzylidene]-3-isopropylidenesuccinic    anhydride, and the like.

The aforementioned process for forming multicolor images can controldensities of colors of the fulgide compounds by controlling themagnitudes of decolorization of the fulgide compounds. Thus, the processcan form multicolor images with wider color reproduction ranges thanconventional equivalents. To apply plural rays of visible radiation withplural wavelengths regions to one area of the image display medium afterbeing colored, these rays may be applied either simultaneously orsubsequently. In the latter case, the rays can be applied in any order.

The photoconductive layer comprises the electron accepting compound andcan thereby form images by controlling the color sensitivity by theapplication of the image forming process, in which the decolorizationsensitivity of the fulgide compounds is temporarily increased uponformation of images and is decreased after the formation of images.Thus, the image display medium and the image forming process can shortenthe time period for image formation and can improve persistence ofcoloring stability of formed images.

Alternatively, the photoconductive layer of the image display medium maybe an assemblage comprising a first photoconductive layer containing thefulgide compound (A) and an electron accepting compound, a secondphotoconductive layer containing the fulgide compound (B) and anotherelectron accepting compound, and a third photoconductive layercontaining the fulgide compound (C) and another electron acceptingcompound.

In the aforementioned image display medium comprising the fulgidecompounds (A), (B), and (C) and one electron accepting compoundsubstantially uniformly contained in the photoconductive layer, changesof the decolorization sensitivity caused by the interaction between theacidic group moieties of the electron accepting compound and thearomatic ring moieties of the fulgide compounds may vary depending onthe electronic properties of the aromatic ring moieties of theindividual fulgide compounds. Specifically, the fulgide compounds mayshow different changes in decolorization sensitivity due to differentelectron donating properties of the aromatic ring moieties of thefulgide compounds.

To avoid this, different electron accepting compounds to yieldappropriate interactions with the fulgide compounds are selected,respectively, first, second, and third photoconductive layers are formedusing, where necessary, a binder material and are laminated to yield animage display medium. By this configuration, changes in thedecolorization sensitivity of even the fulgide compounds (A), (B), and(C) having different electronic properties of their aromatic ringmoieties can be controlled to the substantially same extent, or thechanges in decolorization sensitivity of the fulgide compounds can beoptionally controlled independently.

An index of the selection of a combination of a fulgide compound and anelectron accepting compound in view of the magnitudes of theirinteraction is magnitudes of the electron donating property of thearomatic ring moiety of the fulgide compound and of the electronaccepting property of the acidic group moiety of the electron acceptingcompound. In general, the degree of changes in decolorizationsensitivity increases with the increasing two magnitudes and decreaseswith the decreasing two magnitudes.

The photoconductive layer assemblage preferably further comprisesintermediate layers between the first and second photoconductive layersand between the second and third photoconductive layers. The componentsof the adjacent two layers may be mixed in the vicinity of the interfacebetween the two layers in some processes for the formation of theassemblage. By disposing the intermediate layers, the mixing of thecomponents can be avoided, and the photoconductive layer assemblagecomprising the individual photoconductive layers with set changes indecolorization sensitivity can be obtained.

Materials for the formation of the intermediate layers are preferablytransparent or slightly colored and have some resistance to an organicsolvent used in a coating process, which are preferably employed for theformation of the photoconductive layer. Such preferred materialsinclude, for example, silicone resins, and poly vinylalcohol (PVA). Aprocedure for forming the intermediate layers is not specificallylimited as in the photoconductive layer. The proffered is coating, as itis simple.

The image display media of the present invention may further comprise aprotecting layer on a surface of the photoconductive layer. Theprotecting layer can prevent the functions of the compounds in thephotoconductive layer from deteriorating, can effectively protect theimage display media from mechanical damage and can thereby improve thedurability of the image display media.

Preferred materials for forming the protecting layer are siliconeresins, acrylic resins, poly(vinyl alcohol), and the like, for theirhigh transparency and hardness.

(Multicolor Image-Forming Apparatus)

An image display medium for use in the multicolor image-formingapparatus of the present invention are image display media eachcomprising a photoconductive layer containing at least one photochromiccompound disposed on a substrate. Examples of the image display mediaare image display media each comprising a substrate and aphotoconductive layer disposed on the substrate, which photoconductivelayer comprises two or more photochromic compounds having differentmaximum absorption wavelengths when the fulgide compounds are coloredand at least one electron accepting compound. The image display mediaare preferably the image display media of the present invention.

Such an image display medium is subjected to: a step for applyingultraviolet radiation to the image display medium to thereby color ofall photochromic compounds contained in the photoconductive layer; afirst heating step for heating the image display medium to a meltingpoint of the electron accepting compound or higher; a process forapplying visible radiation having a wavelength corresponding to themaximum absorption wavelength of at least one of the coloredphotochromic compounds to thereby selectively decolorize the at leastone photochromic compound; and a second heating step for heating theimage display medium to the melting point of the electron acceptingcompound or lower. Thus, the decolorization sensitivity can becontrolled. In other words, the decolorization sensitivity of thephotochromic compounds can be temporarily increased during the formationof images and can be decreased after the formation of images.

The image display media for use in the present invention, which comprisethe photoconductive layer comprising the photochromic compounds disposedon the substrate, can form images by the process for forming an image,with changing the decolorization sensitivity. The multicolorimage-forming apparatus of the present invention, which forms images onthe image display medium, will be described with reference to thedrawings.

FIG. 3 is a schematic view showing an example of the multicolorimage-forming apparatus of the present invention.

The multicolor image-forming apparatus of the present invention is anapparatus to form a color image on the image display medium 1. Themulticolor image-forming apparatus accommodates an ultraviolet radiationirradiator 3 configured to irradiate ultraviolet radiation so as tocolor all types of photochromic compounds in a photoconductive layer, afirst heater 4 a configured to temporarily heat the image displaymedium, a visible radiation irradiator 6 configured to irradiate visibleradiation to the photochromic compounds, at a wavelength correspondingto the maximum absorption wavelength of the photochromic compounds so asto selectively decolorize the photochromic compounds, and a secondheater 4 b configured to temporarily heat the image display medium afterirradiating the visible radiation.

Examples of the ultraviolet radiation irradiator 3 configured toirradiate ultraviolet radiation includes, for example, an ultraviolet(UV) lamp, ultraviolet radiation having a suitable wavelength, and thelike. Of those, the ultraviolet radiation having a suitable wavelengthis obtained by combining an optical filter with a mercury lamp, a xenonlamp, or the like.

The examples further include light-emitting device that irradiatesradiation with a specific wavelength, such as an LED.

Examples of the first heater 4 a include heaters such as heat rollers,thermal heads, halogen heaters, ceramic heaters, silica tube heaters,and the like. The first heater 4 a is controlled so that thephotoconductive layer is temporarily heated to Temperature I, which isequal to or higher than the melting point of the electron acceptingcompound, to thereby enable the image display medium 1 to be in State A.

Examples of the visible radiation irradiator 6 include light sources 6 asuch as lamps and LEDs emitting light in wavelengths corresponding tothe absorption wavelengths of the visible range of the photochromiccompounds when the photochromic compounds are colored, andtwo-dimensional light modulation elements 6 b that can controltransmission or reflection of visible radiation in a size correspondingto each pixel of an image to be formed, such as liquid crystal displaypanels and digital micro mirror devices (Dads). Such lamps can be usedin combination with optical filters. A system can also be employed asthe visible radiation irradiator s 6, in which a beam of laser light orLED having an irradiation spot corresponding to each pixel of the targetimage is applied to the image display medium 1 while relatively moving.

Each of the colored photochromic compounds in the image display medium 1can be selectively decolorized by the visible radiation irradiator 6 asshown in FIG. 1B.

Examples of the second heater 4 b are heat rollers, thermal heads,halogen heaters, ceramic heaters, silica tube heaters, and other heatersas in the first heater 4 a. The second heater 4 b is controlled so thatthe photoconductive layer is temporarily heated to Temperature II whichis equal to or lower than the melting point of the electron acceptingcompound to thereby enable the image display medium 1 to be in State B.

The multicolor image-forming apparatus may comprise a heater or a deviceserving both as the first heater 4 a configured to temporarily heat thecolored image display medium 1, and the second heater 4 b configured totemporarily heat the image display medium after irradiating the visibleradiation. In this case, the apparatus requires a system or a mechanismfor switching temperatures so as to enable the heating procedures atdifferent appropriate temperatures in the first and second heatingsteps.

In the formation of color images by the four steps, the image displaymedium 1 is required to be moved to an appropriate position of each ofthe irradiators and the heaters at each of the steps. In addition, it isrequired that the steps be carried out by changing or adjusting theposition at each of the steps, except for a constitution where the imagedisplay medium goes through all of the steps at once.

The multicolor image-forming apparatus of the present invention mayfurther comprise a transfer 5 for the image display medium 1. Thetransfer 5 moves the image display medium 1 relatively to theultraviolet radiation irradiator 3, the first heater 4 a, the visibleradiation irradiator 6, and the second heater 4 b, in this order.Herein, moving the image display medium “relatively” means a case wherethe transfer 5 moves the image display medium 1, or a case where theheaters and the irradiators move the image display medium. The “movingthe image display medium relatively” further includes both of the cases.Thus, the apparatus can automatically move the image display medium 1among the irradiators and the heaters used in the steps, and can controlthe position of the image display medium 1 according to necessity in thesteps.

Alternatively, the transfer 5 for the image display medium 1 may serveso as to transfer the image display medium 1 with respect to theultraviolet radiation irradiator 3, the heater 4, and the visibleradiation irradiator 6 in this order, and then to transfer the imagedisplay medium 1 again to the heater 4 to thereby form images.

More specifically, one heater is used both as the first heater 4 a andthe second heater 4 b, and the heating temperature of the heater isswitched between the first heater and the second heater.

By using a heater as both of the two heaters, the apparatus can reduceits cost and size.

As the transfer 5, for example, a transport roller of roller structuremade of a rubber material used in, for example, copiers and printers canbe used.

The multicolor image-forming apparatus of the present invention mayfurther comprise an inlet 2 a and an outlet 2 b for the image displaymedium 1. The inlet 2 a and the outlet 2 b are capable of automaticallytransferring the image display medium 1 inside the apparatus, the imagedisplay medium 1 inserted into the apparatus through the inlet 2 a,subjecting the image display medium 1 to image forming procedures by theprocesses, and ejecting it from the apparatus through the outlet 2 b. Bythis configuration, the apparatus increases its user friendliness andcan reliably carry out the treatments in the processes.

Alternatively, the apparatus may comprise one inlet/outlet port 2, whichis capable of automatically transporting the image display medium 1inserted into the apparatus through the inlet/outlet port 2, subjectingthe image display medium 1 to image forming procedures by the processes,and ejecting it from the apparatus through the inlet/outlet port 2. Bythis configuration, the apparatus further increases its userfriendliness and can be further miniaturized.

The multicolor image-forming apparatus on the image display medium ofthe present invention may further comprise a white radiation irradiatorto decolorize the entire image display medium.

FIG. 5 is a schematic view of the multicolor image-forming apparatus ofthe present invention, which further includes a white radiationirradiator 9.

To form another image on an image display medium already carrying animage, the multicolor image-forming apparatus of the present inventioncan repeatedly directly form such another image in the image displaymedium without extra decolorizing procedure. However, to decolorize theentire image display medium, the apparatus may further include the whiteradiation irradiator 9 in addition to the visible radiation irradiatorused in the step of selectively decolorizing and irradiates whiteradiation to the entire image display medium, so as to decolorize theentire image display medium in a short time.

The white radiation irradiator 9 can be disposed at such a position asshown in FIG. 6 in the apparatus including the inlet/outlet port 2, butits position is not specifically limited.

EXAMPLES

The present invention will be described in further detail with referenceto several examples below, which are not intended to limit the scope ofthe present invention.

Example A-1

A coating solution was prepared by using 20 parts by weight of2-[1-(5-methyl-2-p-dimethylaminophenyl-4-oxazolyl)ethylidene]-3-isopropylidenesuccinic anhydride (hereinafter briefly referred to as “PC2”) as a photochromic compound, 30 parts by weight oftetradecylphosphonic acid as an electron accepting compound, 50 parts byweight of polystyrene as a binder, and an appropriate amount of tolueneas a solvent. The coating solution was applied to a quartz substrate andthereby yielded a cast film.

The cast film was temporarily heated to 80° C. using a heat roller. Anabsorption spectrum of the heated cast film before irradiation withlight was determined to find that the film showed absorption in rangesfrom 300 nm to a little under 400 nm with a maximum absorptionwavelength of 320 nm and was colorless.

The film was then irradiated with ultraviolet radiation of 366 nmextracted from a high pressure mercury lamp and thereby turned magentawith maximum absorption wavelength of 525 nm in its absorption spectrum.The film was temporarily heated to 170° C. using the heat roller andthereby changed its color to reddish with maximum absorption wavelengthof 485 nm in its absorption spectrum.

The film was then temporarily heated to 80° C. using the heat roller andthereby turned magenta again with maximum absorption wavelength of 525nm in its absorption spectrum.

Next, a cast film having the same composition as above was formed on awhite poly(ethylene terephthalate) (PET) substrate 188 μm thick, a PVAfilm 2 μm thick was formed thereon as a protecting layer and therebyyielded an image display medium. The thus-prepared photoconductive layerwas colorless and was formed on the white substrate, and the resultingimage display medium was seen white by a viewer.

The photoconductive layer of the image display medium was temporarilyheated to 80° C. using a heat roller, was irradiated with ultravioletradiation of 366 nm to thereby saturate a color reaction and wasirradiated with visible radiation with a center wavelength of 520 nm anda half-width of 10 nm at an illuminance of 1 mW/cm² for decolorization.In this procedure, an irradiation time period of the visible radiationwas changed, and changes in reflection spectra were determined. FIG. 7is a view showing an example of changes in reflection spectrum withirradiation time. The reflection spectrum changed with irradiation timeperiod of the visible radiation from saturated colored state 12 tosaturated decolorized state 13. In FIG. 7, the arrow means a directionfrom a short irradiation time period to a long one. Next, an illuminancewas calculated from the irradiation time period of the visible radiationto thereby determine the relationship between the illuminance and thereflection ratio. FIG. 8 is a graph showing the relationship between theilluminance and the reflection ratio at a bottom wavelength. Anilluminance to make the change in reflection ratio at the bottomwavelength 90% or more of the total changes was determined based on FIG.7 and was found to be about 1500 mJ/cm². The thus-determined illuminancewas defined as the decolorizing energy as an index of the decolorizationsensitivity.

The photoconductive layer of the image display medium was irradiatedagain with ultraviolet radiation of 366 nm to thereby saturate a colorreaction, was temporarily heated to 170° C. using the heat roller, andwas irradiated with visible radiation with a center wavelength of 480 nmand a half-width of 10 nm at an illuminance of 1 mW/cm². Thedecolorizing energy in this procedure was determined in the same manneras above and was found to be 20 mJ/cm². The photoconductive layer of theimage display medium was irradiated again with ultraviolet radiation of366 nm to thereby saturate a color reaction, was temporarily heated to80° C. using the heat roller, and was irradiated with visible radiationwith a center wavelength of 520 nm and a half-width of 10 nm at anilluminance of 1 mW/cm². The decolorizing energy in this procedure wasdetermined in the same manner as above and was found to be about 1500mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a phosphonic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-2

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of alpha-hydroxytetradecanoic acid as theelectron accepting compound, 40 parts by weight of polystyrene, and 20parts by weight of PC 2 were used. The image display medium wassubjected to heat treatments and its decolorizing energy was determinedby the procedures of Example A-1. The decolorizing energy aftertemporarily heating to 80° C. was about 1500 mJ/cm² as in Example A-1,and the energy after temporarily heating to 170° C. was 33 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a carboxylic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-3

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of 2-fluorooctadecanoic acid as theelectron accepting compound, 40 parts by weight of polystyrene, and 20parts by weight of PC 2 were used. The image display medium wassubjected to heat treatments and its decolorizing energy was determinedby the procedures of Example A-1. The decolorizing energy aftertemporarily heating to 80° C. was about 1500 mJ/cm² as in Example A-1and that after temporarily heating to 170° C. was 35 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a carboxylic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer with.

Example A-4

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of 2-oxooctadecanoic acid as the electronaccepting compound, 40 parts by weight of polystyrene, and 20 parts byweight of PC 2 were used. The image display medium was subjected to heattreatments and its decolorizing energy was determined by the proceduresof Example A-1. The decolorizing energy after temporarily heating to 80°C. was about 1500 mJ/cm² as in Example A-1 and that after temporarilyheating to 170° C. was 35 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a carboxylic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-5

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of 2-(octadecylthio)succinic acid as theelectron accepting compound, 40 parts by weight of polystyrene, and 20parts by weight of PC 2 were used. The image display medium wassubjected to heat treatments and its decolorizing energy was determinedby the procedures of Example A-1. The decolorizing energy aftertemporarily heating to 80° C. was about 1500 mJ/cm² as in Example A-1and that after temporarily heating to 170° C. was 22 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a carboxylic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-6

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of octadecylsuccinic acid as the electronaccepting compound, 40 parts by weight of polystyrene, and 20 parts byweight of PC 2 were used. The image display medium was subjected to heattreatments and its decolorizing energy was determined by the proceduresof Example A-1. The decolorizing energy after temporarily heating to 80°C. was about 1500 mJ/cm² as in Example A-1 and that after temporarilyheating to 170° C. was 33 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a carboxylic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-7

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of octadecylmalonic acid as the electronaccepting compound, 40 parts by weight of polystyrene, and 20 parts byweight of PC 2 were used. The image display medium was subjected to heattreatments and its decolorizing energy was determined by the proceduresof Example A-1. The decolorizing energy after temporarily heating to 80°C. was about 1500 mJ/cm² as in Example A-1 and that after temporarilyheating to 170° C. was 28 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a carboxylic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-8

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of 2-octadecylglutaric acid as theelectron accepting compound, 40 parts by weight of polystyrene, and 20parts by weight of PC 2 were used. The image display medium wassubjected to heat treatments and its decolorizing energy was determinedby the procedures of Example A-1. The decolorizing energy aftertemporarily heating to 80° C. was about 1500 mJ/cm² as in Example A-1and that after temporarily heating to 170° C. was 30 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a carboxylic acid compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-9

An image display medium was prepared by the procedure of Example A-1,except that 40 parts by weight of p-(octadecylthio)phenol as theelectron accepting compound, 40 parts by weight of polystyrene, and 20parts by weight of PC 2 were used. The image display medium wassubjected to heat treatments and its decolorizing energy was determinedby the procedures of Example A-1. The decolorizing energy aftertemporarily heating to 80° C. was about 1500 mJ/cm² as in Example A-1and that after temporarily heating to 170° C. was 40 mJ/cm².

These results verify that the decolorization sensitivity can bereversibly changed by incorporating a phenolic compound having analiphatic group containing 12 or more carbon atoms as the electronaccepting compound into the photoconductive layer.

Example A-10

As photochromic compounds,2-[1-(5-methyl-2-phenyl-4-oxazolyl)ethylidene]-3-isopropylidenesuccinicanhydride (hereinafter briefly referred to as “PC 1”), PC 2, and2-[1-(1,2,5-trimethyl-3-pyrrolyl)ethylidene]-3-isopropylidenesuccinicanhydride (hereinafter briefly referred to as “PC 3”) were used. Acoating solution was prepared on a quartz substrate using 30 parts byweight of PC 1, 30 parts by weight of tetradecylphosphonic acid as anelectron accepting compound, 40 parts by weight of polystyrene as abinder, and an appropriate amount of toluene as a solvent, and a castfilm was prepared on a quartz substrate using the coating solution. Twocast films using PC 2 and PC 3 instead of PC 1, respectively, wereformed on a quartz substrate in the same manner as above.

These cast films were temporarily heated to 80° C. using a heat roller.The absorption spectra of the cast films using PC 1, PC 2, or PC 3before irradiation with light were determined to find that they showedabsorption in ranges from 300 nm to a little below 400 nm with maximumabsorption wavelengths of 337 nm, 320 nm, and 360 nm, respectively, andwere colorless.

The cast films prepared using PC 1, PC 2 or PC 3 were irradiated withultraviolet radiation of 366 nm extracted from a high pressure mercurylamp, turned yellow, magenta, and cyan with maximum absorptionwavelengths of 450 nm, 525 nm, and 660 nm, respectively.

These cast films were temporarily heated to 170° C. using the heatroller, turned colors with maximum absorption wavelengths in absorptionspectra of 420 nm, 485 nm, and 620 nm, respectively. The cast films werethen temporarily heated to 80° C. using the heat roller and turned backcolors with maximum absorption wavelengths in absorption spectra of 450nm, 525 nm, and 660 nm, respectively.

A coating solution was prepared by using 10 parts by weight of PC 1, 10parts by weight of PC 2, 10 parts by weight of PC 3, 30 parts by weightof tetradecylphosphonic acid, 40 parts by weight of polystyrene, and anappropriate amount of toluene as a solvent. A cast film was formed on a188 μm thick white poly ethyleneterephthalate (PET) substrate using thecoating solution, a 2 μm thick PVA film was formed thereon as aprotecting layer. The thus-prepared photoconductive layer was colorlessand was formed on the white substrate, and the resulting image displaymedium was seen white by a viewer.

The photoconductive layer of the image display medium was temporarilyheated to 80° C. using a heat roller and was irradiated with ultravioletradiation of 366 nm. Thus, all of the PC 1, PC 2 and PC 3 colored, andthe image display medium turned black. The photoconductive layer wasthen irradiated with white radiation, became transparent with no coloragain, and the resulting image display medium was seen white by aviewer.

The image display medium was irradiated with ultraviolet radiation of366 nm again, a part of which was then irradiated with visible radiationwith a center wavelength of 450 nm and a half-width of 10 nm. Thus, PC 1in an irradiated portion was selectively decolorized, and the irradiatedportion turned blue. Another portion of the image display medium wasirradiated with visible radiation with a center wavelength of 520 nm anda half-width of 10 nm to thereby selectively decolorize PC 2 and turnedgreen. Yet another portion of the image display medium was irradiatedwith visible radiation with a center wavelength of 660 nm and ahalf-width of 10 nm to thereby selectively decolorize PC 3 and turnedred.

The decolorizing energy of PC 1, PC 2, and PC 3 was determined inunexposed portions to the visible radiation using visible radiation ofthe three wavelengths distributions in the same manner as in Example A-1and was found to be 580 mJ/cm², 1050 mJ/cm², and 790 mJ/cm²,respectively.

The photoconductive layer of the image display medium was irradiatedwith ultraviolet radiation of 366 nm again to thereby saturate a colorreaction and was temporarily heated to 170° C. using the heat roller.The decolorizing energy of PC 1, PC 2, and PC 3 was determined usingvisible radiation of 420 nm, 480 mm, and 620 nm in the same manner asabove and was found to be 20 mJ/cm², 33 mJ/cm², and 26 mJ/cm²,respectively. Thereafter, the photoconductive layer of the image displaymedium was irradiated with ultraviolet radiation of 366 nm again tothereby saturate a color reaction and was temporarily heated to 80° C.using the heat roller. The decolorizing energy of PC 1, PC 2, and PC 3was determined in the same manner as above and was found to be 580mJ/cm², 1050 mJ/cm², and 790 mJ/cm², respectively.

These results verify that the decolorizing energy of individualphotochromic compounds can be reversibly largely changed by simpleheating procedures even in an image display medium having aphotoconductive layer using three photochromic compounds.

Example A-11

An image display medium was prepared in the following manner. A castfilm was formed on a white PET substrate 188 μm thick using a coatingsolution containing 30 parts by weight of PC 1, 30 parts by weight ofp-(octadecylthio)phenol, 40 parts by weight of polystyrene, and anappropriate amount of toluene as a solvent; a PVA intermediate layer wasformed thereon; another cast film was formed thereon using a coatingsolution containing 30 parts by weight of PC 2, 30 parts by weight oftetradecylphosphonic acid, 40 parts by weight of polystyrene, and anappropriate amount of toluene as a solvent; a PVA intermediate layer wasformed thereon; yet another cast film was formed thereon using a coatingsolution containing 30 parts by weight of PC 3, 30 parts by weight of2-octadecylglutaric acid, 40 parts by weight of polystyrene, and anappropriate amount of toluene as a solvent; a PVA protective film wasformed thereon and thereby yielded an image display medium. Thethus-prepared photoconductive layer was colorless, was formed on thewhite substrate, and the resulting image display medium was seen whiteby a viewer.

The photoconductive layer of the image display medium was temporarilyheated to 80° C. using a heat roller and was irradiated with ultravioletradiation of 366 nm. Thus, all PC 1, PC 2, and PC 3 colored and theimage display medium turned black. The photoconductive layer was thenirradiated with white radiation, became transparent and colorless again,and the resulting image display medium was seen white by a viewer.

The image display medium was irradiated with ultraviolet radiation of366 nm again, a part of which was then irradiated with visible radiationwith a center wavelength of 450 nm and a half-width of 10 nm. Thus, PC 1in an irradiated portion was selectively decolorized, and the irradiatedportion turned blue. Another portion of the image display medium wasirradiated with visible radiation with a center wavelength of 520 nm anda half-width of 10 nm to selectively decolorize PC 2 and thereby turnedgreen. Yet another portion of the image display medium was irradiatedwith visible radiation with a center wavelength of 660 nm and ahalf-width of 10 nm to selectively decolorize PC 3 and thereby turnedred.

The decolorizing energy of PC 1, PC 2, and PC 3 was determined inunexposed portions to the visible radiation using visible radiation ofthe three wavelengths distributions in the same manner as in Example A-1and was found to be 580 mJ/cm², 1050 mJ/cm², and 790 mJ/cm²,respectively.

The photoconductive layer of the image display medium was irradiatedwith ultraviolet radiation of 366 nm again to thereby saturate a colorreaction and was temporarily heated to 170° C. using the heat roller.The decolorizing energy of PC 1, PC 2, and PC 3 was determined usingvisible radiation with center wavelengths of 430 nm, 500 nm, and 630 nmin the same manner as above and was found to be 30 mJ/cm², 33 mJ/cm²,and 32 mJ/cm², respectively. Thereafter, the photoconductive layer ofthe image display medium was irradiated with ultraviolet radiation of366 nm again to thereby saturate a color reaction and was temporarilyheated to 80° C. using the heat roller. The decolorizing energy of PC 1,PC 2, and PC 3 was determined in the same manner as above and was foundto be 580 mJ/cm², 1050 mJ/cm², and 790 mJ/cm², respectively.

These results verify that the decolorizing sensitivities of threephotochromic compounds can be controlled at a substantially equal levelas in the present example, by forming three photoconductive layerscontaining three different combinations of the photochromic compoundswith corresponding electron accepting compounds, respectively.

As described above, the present invention can provide image displaymedia and processes for forming images that are rewritable uponirradiation with light, which media and processes can shorten the timeperiod for image formation and can improve persistence of coloringstability of formed images.

Example B-1

An multicolor image-forming apparatus having the configuration shown inFIG. 3 was prepared as an apparatus for forming images on an imagedisplay medium as obtained in Example A-11. The image display medium canchange its decolorization sensitivity by a heat treatment.

The apparatus included a black light with a center wavelength in lightemission of 360 nm as the ultraviolet radiation irradiator; ceramic lineheaters as the heaters with set heating temperatures of 170° C. and 80°C., respectively; a liquid crystal panel (768×1024 pixels) as thetwo-dimensional optical modulator; and three straight-tube light sources11 shown in FIG. 10 with center wavelengths in light emission of 430 nm,500 nm, and 630 nm, respectively, as the visible radiation irradiator.The light sources 11 had wavelength properties in light emission asshown in FIG. 9. The apparatus further included a mechanism forswitching the positions of the light sources 11 by rotation and a lightintegrator 10 for uniformizing the illuminance in the two-dimensionaloptical modulator.

In this apparatus, an image display medium inserted into the apparatusthrough the inlet is transported to a set position on thetwo-dimensional optical modulator at a set speed. During thetransportation, the image display medium is sequentially subjected to acolor development process by irradiation with ultraviolet radiation anda changing (increasing) process of the decolorization sensitivity by thefirst heater. While operating the two-dimensional optical modulatoraccording to input image data, the three visible radiation light sourcesare sequentially switched to subject the image display medium to adecolorizing process by irradiation with visible radiation to therebyform a color image. The image display medium is then transported to theoutlet while subjecting the same to changing (decreasing) of thedecolorization sensitivity by the second heater and is ejected from theapparatus through the outlet.

Using this apparatus, a color image was formed on the image displaymedium according to Example A-11 at a transport speed of 50 mm/sec andan illumination time period of the visible radiation irradiator per onelight source of 7 seconds and thereby yielded a clear color image. Ittook about 30 seconds to perform all the processes. The image displaymedium carrying the thus-formed image was inserted into the inlet of theapparatus again, another image was input to write the image on themedium and thereby yielded a new image without any problem.

Comparative Example B-1

An apparatus having the same configuration as the multicolorimage-forming apparatus in Example B-1, excluding the first and secondheater, was prepared. Using the apparatus, a color image was formed onan image display medium by the procedure of Example B-1. However, thedecolorizing process by irradiation with visible radiation was notperformed sufficiently under the same conditions as in Example B-1, andthe apparatus failed to form a sharp color image. To form a sharp colorimage using this apparatus, it took four minutes or more to decolorizethe image display medium by irradiation with visible light.

Example B-2

An multicolor image-forming apparatus having the configuration shown inFIG. 4 was prepared. The apparatus included one heater serving both asthe first and second heater in the apparatus according to Example B-1and was capable of switching the heating temperature between 170° C. and80° C. The apparatus included the same ultraviolet radiation irradiatorand the visible radiation irradiator as in Example B-1. The apparatusserved to perform the same processes until the decolorizing process byirradiation with visible radiation as in Example B-1, but served totransport the image display medium in an opposite direction, to switch(decrease) the decolorization sensitivity by the heater with a heatingtemperature set at 80° C. and to eject the medium from the apparatusthrough the inlet/outlet port. This apparatus can reduce its size andcost as compared with the apparatus according to Example B-1. Using theapparatus, a sharp color image could be formed on an image displaymedium as in Example B-1.

Example B-3

An multicolor image-forming apparatus having the configuration shown inFIG. 5 was prepared. The resulting apparatus further included a whiteradiation irradiator in addition to the configuration of the apparatusaccording to Example B-1. The apparatus could erase an entire image ofan image display medium carrying the formed image in a short time whilecontinuously transporting the medium from the inlet. In this procedure,the apparatus did not require a decolorizing process by the visibleradiation irradiator and thereby did not stop the transport of the imagedisplay medium at the position of the visible radiation irradiator.

Example B-4

An multicolor image-forming apparatus having the configuration shown inFIG. 6 was prepared. The resulting apparatus further included a whiteradiation irradiator in addition to the configuration of the apparatusaccording to Example B-2. The apparatus could erase an entire image ofan image display medium carrying the formed image in a short time whilecontinuously transporting the medium from the inlet. In this procedure,the apparatus did not require a decolorizing process by the visibleradiation irradiator and thereby did not stop the transport of the imagedisplay medium at the position of the visible radiation irradiator.

As is described above, the present invention can provide the multicolorimage-forming apparatus, which can control its decolorizationsensitivity, can shorten the image forming time period and can ensurethe persistence of coloring stability of formed images.

1-19. (canceled)
 20. A multicolor image forming apparatus comprising: anultraviolet radiation irradiator configured to irradiate ultravioletradiation to an image display medium so as to color all types ofphotochromic compounds contained in a photoconductive layer; a firstheater configured to temporarily heat the image display medium; avisible radiation irradiator configured to irradiate visible radiationto the image display medium, at wavelength corresponding to the maximumabsorption wavelength of each of the photochromic compounds in a stateof colored so as to selectively decolorize the photochromic compounds;and a second heater configured to temporarily heat the image displaymedium after irradiating the visible radiation, wherein the imagedisplay medium repeatedly forms a multicolor image, and the imagedisplay medium comprises: a photoconductive layer containing two or moretypes of photochromic compounds and an electron accepting compound; anda substrate, wherein the photochromic compounds have different maximumabsorption wavelengths when colored.
 21. A multicolor image-formingapparatus according to claim 20, further comprising a transferconfigured to transfer the image display medium, wherein the imagedisplay medium relatively moves towards the ultraviolet radiationirradiator, the first heater, the visible radiation irradiator, and thesecond heater, in this order.
 22. A multicolor image-forming apparatusaccording to claim 21, further comprising an inlet and an outlet,wherein the image display medium is inserted from the inlet, the imagedisplay medium is automatically transferred inside the multicolorimage-forming apparatus, and the image display medium is ejected fromthe outlet.
 23. A multicolor image forming apparatus comprising: anultraviolet radiation irradiator configured to irradiate ultravioletradiation to an image display medium so as to color all types ofphotochromic compounds contained in a photoconductive layer; a visibleradiation irradiator configured to irradiate visible radiation to adesired portion of the image display medium, at wavelength correspondingto the maximum absorption wavelength of each of the photochromiccompounds in a state of colored so as to selectively decolorize thephotochromic compounds; and a heater configured to temporarily heat theimage display medium, wherein the image display medium repeatedly formsa multicolor image, the heater heats the image medium both before andafter irradiating visible radiation, after irradiating ultravioletradiation, and the image display medium comprises: a photoconductivelayer containing two or more types of photochromic compounds and anelectron accepting compound; and a substrate, wherein the photochromiccompounds have different maximum absorption wavelengths when colored.24. A multicolor image-forming apparatus according to claim 23, furthercomprising a transfer configured to transfer the image display medium,wherein the image display medium relatively moves towards theultraviolet radiation irradiator, the heater, and the visible radiationirradiator, in this order, and, the image display medium moves back tothe heater.
 25. A multicolor image-forming apparatus according to claim24, further comprising an inlet/outlet port which works as an inlet toinsert an image display medium and as an outlet to eject the imagedisplay medium, wherein the image display medium is automaticallytransferred from the inlet/outlet to inside the multicolor image-formingapparatus, and the image display medium is ejected from theinlet/outlet.
 26. A multicolor image-forming apparatus according toclaim 20, further comprising a visible radiation irradiator configuredto irradiate white light to the image display medium so as to decolorizean entire desired portion of the image display medium.